docs: def com_google_fonts_check_metadata_regular_is_400(family_metadata):
    """METADATA.pb: Regular should be 400."""
    badfonts = []
    for f in family_metadata.fonts:
      if f.full_name.endswith("Regular") and f.weight != 400:
        badfonts.append(f"{f.filename} (weight: {f.weight})")
    if len(badfonts) > 0:
      yield FAIL, ("METADATA.pb: Regular font weight must be 400."
                   " Please fix these: {}").format(", ".join(badfonts))
    else:
      yield PASS, "Regular has weight = 400."